Difference between revisions of "Design Features"

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(High Draft)
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* Rationale: increased stability by averaging out wave effects over a long span
 
* Rationale: increased stability by averaging out wave effects over a long span
 
* Mechanism: 'being big' is the proven method for increasing stability out on the ocean. There are two attractive aspects to having a big footprint, pertaining to roll and heave.
 
* Mechanism: 'being big' is the proven method for increasing stability out on the ocean. There are two attractive aspects to having a big footprint, pertaining to roll and heave.
** With respect to roll: a wider structure has a more favorable metacentric height: any attempt to roll it over results in a large restoring force, which leads to smaller rolling motions.
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** Roll: a wider structure has a more favorable metacentric height: any attempt to roll it over results in a large restoring force, which leads to smaller rolling motions. Compare the roll and pitch motions of a ship; because of its elongated shape, it would much sooner roll than pitch.
** With respect to heave: the upward forcing effect of the water and its waves is averaged out over a larger area, meaning the net heaving force
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** Heave: the upward forcing effect of the water and its waves is averaged out over a larger area, meaning the net heaving motions are reduced.
 
* Drawbacks:  
 
* Drawbacks:  
** A big footprint implies a big structure. In order for size to start to matter against oceanic waves, quite some size is needed. 20m is still small in ocean waves. Compare the roll and pitch motions of a ship; because of its elongated shape, it would much sooner roll than pitch.
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** A big footprint implies a big structure. In order for size to start to matter against oceanic waves, quite some size is needed. 20m is still small in ocean waves.  
 
** Bigger means more fragile. The bigger a structure is, the bigger forces it can bring down upon itself. Driftwood doesnt break in a storm; boats do. Big boats need to get out of the way in big storms, or they run a risk of catastrophic damage (reference miguels presentation).
 
** Bigger means more fragile. The bigger a structure is, the bigger forces it can bring down upon itself. Driftwood doesnt break in a storm; boats do. Big boats need to get out of the way in big storms, or they run a risk of catastrophic damage (reference miguels presentation).
  

Revision as of 19:49, 14 October 2009

Seastead design features

Many designs for ocean going structures exists, and many have been suggested for the explicit purpose of seasteading. There is significant overlap between these different designs: often, these differences can be viewed as mere variations along a continuous spectrum of some parameter.

This section is an attempt at identifying the main features or characteristics a design might employ to meet the challenges of providing a safe and comfortable piece of real-estate. In this way, we can look at seastead concepts in a more systematic way, avoid reinventing the wheel, and quickly get a qualitative feel for the properties of a given design. Not all designs can be perfectly categorized in such a way, but it creates some order in the chaos nonetheless.


High Draft

  • Examples: FLIP, spar platform
  • Rationale: improved stability
  • Mechanism: The mechanism by which a spar gains its exceptional stability is twofold
    • Intertial: its high mass makes it hard to move, and its elongated shape makes it hard to roll over. Because the ballast is so deep, the center of gravity is strictly below the center of buoyancy, which gives it unconditional stability.
    • Loading: wave phenomena are biased towards the surface of the water (effects fall off exponentially with depth). Hence, a deep spar is resting largely in stationary waters, and is only being pushed around at the top. The pressure fluctuations that drive heave motions hardly make it to the bottom at all, hence the heave-inducing vertical force fluctuations are small.
  • Drawbacks: High draft. This complicates deployment in deep waters, and rules out operation in shallow waters. It is not clear that the concept scales down to a more incremental size. It has never been done before (spar: 200m-ish, FLIP 100m-ish), and its operating mechanism suggests it will lose its stability properties once scaled down.

Big Footprint

  • Examples: cruiseship, clubstead, pontoon
  • Rationale: increased stability by averaging out wave effects over a long span
  • Mechanism: 'being big' is the proven method for increasing stability out on the ocean. There are two attractive aspects to having a big footprint, pertaining to roll and heave.
    • Roll: a wider structure has a more favorable metacentric height: any attempt to roll it over results in a large restoring force, which leads to smaller rolling motions. Compare the roll and pitch motions of a ship; because of its elongated shape, it would much sooner roll than pitch.
    • Heave: the upward forcing effect of the water and its waves is averaged out over a larger area, meaning the net heaving motions are reduced.
  • Drawbacks:
    • A big footprint implies a big structure. In order for size to start to matter against oceanic waves, quite some size is needed. 20m is still small in ocean waves.
    • Bigger means more fragile. The bigger a structure is, the bigger forces it can bring down upon itself. Driftwood doesnt break in a storm; boats do. Big boats need to get out of the way in big storms, or they run a risk of catastrophic damage (reference miguels presentation).

Sparse Footprint

  • Examples: Catamaran, Minifloat, WaterWalker
  • Rationale: benefit of a large effective footprint, with minimum material use.
  • Mechanism: essentially the same arguments that apply to a big footprint: A wider structure is more resistant to rolling motions. Instead of having one big hull, connecting a few small hulls by trusses spanning the same area, has roughly the same stability benefits, while being much more scale-friendly.
  • Drawbacks: not as modular as it seems. One can rigidly connect three units in a triangle without any problems, but growing this structure further brings back the fragility problems of a big structure.

Minimize waterplane area

  • Examples: spar platforms, semi-submersibles, clubstead, FLIP
  • Rationale: minimize interactions with the waves.
  • Mechanism: Floating vessels derive their stable position on the water from the fact that moving them up and down changes the volume of displaced water. If there is little waterplane area, this coupling is weak. This means the waves will have relatively little effect. On the other hand, the total flotation has to come from somewhere; as can be seen from any of the examples, this implies that the flotation is located somewhere below the waterline. This necessarily implies a medium to high draft, which has significant drawbacks (see: Design requirements/incrementalism/draft). Roll-stability can derive from any source; through the use of very deep ballast (FLIP, Spar), or wide footprint (semi-sub), or a combination of both (clubstead)
  • Drawbacks:
    • Only works up to a given waveheight. How big of an air-gap do you design for? Being relatively unaffected by waves up to 10m is great; but how will you handle the 5m of a 15m wave that will hit your platform?
    • Poorly compatible with small scale designs. A 20m airgap is hard to fit into a structure that is supposed to be small, especially considering the volume of underwater construction needed to obtain stability.

Shock Absorbers

  • Examples: Pneumatically Stabilized Platform
  • Rationale: cushion wave forces
  • Mechanism: Anything near the surface of the water will be moved around by waves with hardly stoppable force. Yet we need flotation. Instead of rigidly connecting your platform with these vigorously moving waves, one can connect them by means a spring of sorts. A practical implementation of this seemingly overcomplicated idea is found in the Pneumatically Stabilized Platform. By floating a cylinder with an open bottom end in the water, the entrapped compressible air will naturally act as a spring between the water surface and the top-end of the cylinder.
  • Drawbacks:
    • still an experimental idea, that has never been applied and tested in a real application.
    • same objection as minimizing water plane area. What air-gap do you choose, and how to avoid catastrophe in case it is exceeded?
    • Good stability demands a large platform.

Heave plates

  • Examples: spar platforms, semi-submersibles, clubstead
  • Rationale: increase the difficulty of moving the structure up and down.
  • Mechanism: heave plates increase the amount of water that needs to move in order for the structure to be able to move. This increases the added mass (link), which improves overall performance. As opposed to minimizing waterplane area or other strategies to minimizing coupling with the wavy surface waters, this techniques aims to increase coupling with the relatively stationary deep waters.
  • Drawbacks: need to be located in relatively deep waters, less affected by wave motion. Adding them at the surface would only increase the tendency of your structure to follow the waves.

Conclusions

Most design features aimed at providing stability in strong waves fail to scale down well.